storage: fix retry strategy to unwrap errors (#4019)

I updated retry strategy in google-cloud-go to follow what [google-api-go-client](https://github.com/googleapis/google-api-go-client/blob/e8cf7f70a1e0160405dd0f789a006e6b6074fbdb/internal/gensupport/resumable.go#L233-L258) by unwrapping errors which can be unwrapped. 

For testing, I followed wrapping guidance in [godocs](https://golang.org/pkg/errors/#pkg-overview) to verify the change works as expected.

Thank you in advance for your review.

Fixes: #3963

storage/go.mod

@@ -8,6 +8,7 @@ require (
 	github.com/google/go-cmp v0.5.5
 	github.com/googleapis/gax-go/v2 v2.0.5
 	golang.org/x/oauth2 v0.0.0-20210413134643-5e61552d6c78
+	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
 	google.golang.org/api v0.45.0
 	google.golang.org/genproto v0.0.0-20210427215850-f767ed18ee4d
 	google.golang.org/grpc v1.37.0

storage/go110.go

@@ -43,10 +43,14 @@ func shouldRetry(err error) bool {
 				return true
 			}
 		}
-		return false
 	case interface{ Temporary() bool }:
-		return e.Temporary()
-	default:
-		return false
+		if e.Temporary() {
+			return true
+		}
+	}
+	// Unwrap is only supported in go1.13.x+
+	if e, ok := err.(interface{ Unwrap() error }); ok {
+		return shouldRetry(e.Unwrap())
 	}
+	return false
 }

storage/go110_test.go

@@ -23,6 +23,8 @@ import (
 	"net/url"
 	"testing"
 
+	"golang.org/x/xerrors"
+
 	"google.golang.org/api/googleapi"
 )
 
@@ -45,6 +47,8 @@ func TestInvoke(t *testing.T) {
 		{2, &googleapi.Error{Code: 599}, &googleapi.Error{Code: 428}},
 		{1, &url.Error{Op: "blah", URL: "blah", Err: errors.New("connection refused")}, nil},
 		{1, io.ErrUnexpectedEOF, nil},
+		{1, xerrors.Errorf("Test unwrapping of a temporary error: %w", &googleapi.Error{Code: 500}), nil},
+		{0, xerrors.Errorf("Test unwrapping of a non-retriable error: %w", &googleapi.Error{Code: 400}), &googleapi.Error{Code: 400}},
 	} {
 		counter := 0
 		call := func() error {